Devices herein generally relate to xerographic or electrostatographic printing machines, and more particularly to a drum unit and a grounding device used in the drum unit, which are used in image forming apparatuses, such as copying machines, laser printers, and facsimile machine.
In an image forming apparatus, for example a copying machine, an original image or document is usually read by an exposure section to form an electrostatic latent image on a photosensitive drum. A developing unit for forming toner image is disposed about an outer circumference of the photosensitive drum. The developing unit develops the read image by charging toner supplied from a toner hopper so that it has an electrostatic charge that is opposite that of the electrostatic latent image on the photosensitive drum. The toner adheres to the photosensitive drum at oppositely charged portions of the drum corresponding to the electrostatic latent image through a developing sleeve.
Electrophotographic imaging members are well known in the art. One type of photoreceptor conventionally utilized for copiers and printers and the like comprises a hollow photosensitive drum. The drum includes a tube that is obtained by forming a conductive metal into a cylindrical shape. Typically, the drum has been dip coated with various coatings including at least one photosensitive coating. These photoreceptors are usually supported on an electrically conductive shaft by drum supporting hubs or end flanges. The hubs can be constructed of metal, which is very robust to fatigue effects and deformation due to interference with the drive shaft. Hubs provide excellent stability and efficient grounding over the part life, however these parts are not cost effective. Hubs made of plastic material are lower cost and have a hole through their center into which a supporting axle shaft is inserted. That is, a flange member is forced into openings at opposing ends of the drum tube to provide the interface to a driving mechanism for rotation of the drum. Hubs constructed of metal provide a direct grounding connection to the metal substrate, while hubs constructed of electrically insulating plastic material require an electrical grounding device, which can be secured to the hub and positioned to contact both the electrically conductive axle shaft and the electrically conductive metal substrate of the photosensitive drum.
To produce a conductive state between the drum tube and the apparatus body, an electrical grounding device, sometimes referred to as an earth plate or ground plate, may be attached to the flange member. The ground plate part provides a ground path from the photosensitive drum tube through the drive shaft. The ground plate has an outer peripheral contact portion that is configured to contact the drum tube. An inner contact portion of the ground plate is configured to be in contact with the periphery of the drive shaft.
Ground plate devices can be constructed of various metals including, aluminum, copper, bronze, stainless steel, and the like. Material selection is key to providing a good reliable contact with the shaft, to be robust against corrosion in various environments, and withstand frictional stress during machine operation. Any intermittent loss of contact between the ground plate and the drive shaft will result in image quality defects on the printed output. Some ground plate devices are susceptible to this type of defect due to misalignment of the motor drive shaft. The misalignment induces a wobble type motion of the drive shaft relative to the ground plate and causes the ground plate contact pieces to vibrate or slide back and forth against the drive shaft. This relative surface motion, at an interface, which is supposed to be static, induces fretting corrosion, which can impede electrical continuity, even with good contact, which then results in image defects, as well as inducing other electrical noise issues within the image forming apparatus.
Therefore, a need exists for a grounding device that can maintain electrical contact with the drive shaft in the event of misalignment and avoids metal-to-metal friction with the drive shaft.